I once bought some s/h computers from a famous college in London via a "refurbishing" company; the HDDs were chock full of student data.
(The drives were 95% full - the performance slow down is probably why the idiots sold them off).
As far as I could tell, "refurbishing" meant using a damp cloth to wipe the dust off of the cases; as they hadnt even done a basic reformat of the HDDs.
If I had been Dr Evil, I could have re-cooped my £40 per box many times over.